Turbine engine blades, related articles, and methods

ABSTRACT

A compressor blade generally used in turbine engines is presented. The compressor blade includes a protective covering bonded to a tip portion of the blade with a braze material. The braze material includes from about 1 weight percent to about 10 weight percent of an active metal element, based on the total amount of the braze material. A compressor rotor is also provided that includes a plurality of the compressor blades. A method for joining a protective covering to a tip portion of a compressor blade, and a method for repair of a compressor blade, are also provided.

BACKGROUND OF THE INVENTION

This invention relates generally to turbine blades, and moreparticularly to compressor blades of a turbine engine. In some specificembodiments, the invention relates to hard tips for compressor blades,related articles, and methods for joining the hard tips to tip portionsof the compressor blades.

Typically, gas turbine engines are comprised of three major sections orcomponents which function together to produce thrust for propulsion, orenergy for generating power. The compressed air from the compressorsection is delivered to a combustion section where fuel is added to theair and ignited, and thereafter delivered to the turbine section of theengine, where a portion of the energy generated by the combustionprocess is extracted by a turbine to drive the engine compressor.

Each turbine section includes a rotor assembly comprising a plurality ofrotor blades, each extending radially outward from a disk across theairflow path. More specifically, each rotor blade has a dovetail whichengages with the disk, and an airfoil extending radially from the diskto a blade tip at the opposite end. In the initial sections of theengine, the compressor blades are generally made of titanium alloys, ora martensitic stainless steel, and in the later sections, the highpressure turbine blades are generally made of ferrous or nickel basealloys.

A shroud encompasses the blade tips with as little radial gap(clearance) as possible, in order to minimize bypass flow of air orother gases past the tips of the blades. The purpose of the narrow gapis to minimize gas leakage and to allow the pressure of the air toincrease from one section or stage to the next. A narrower gap (orreduced clearance) between the tips and the adjacent shroud usuallyincreases the engine efficiency and power output of the engine. Forexample, every 10 mil decrease in the clearance in the hot gas path cansignificantly improve the power output of the turbine engines.

The minimization of the gap is often limited by several factors, forexample, manufacturing tolerances, differing rates of thermal expansion,mass inertia effect, and dynamic effects. If the gap is too narrow,there is the possibility of undesirable contact (e.g., rubbing) betweenthe tip and the shroud. In these cases, the blade can heat up fasterthan the surrounding shroud, and thus come into contact with the shroud,due to thermal expansion and mass inertia differentials. There arelikely other mechanisms (e.g., deformation, oxidation) that also causethis contact (e.g., rubbing), and in turn, damage or unduly wear theblade tips. For example, the inner diameter of the shroud is not usuallyconcentric with the axis of rotation of the blades, and the blade tipscan rub the surrounding shroud. Damage to the blade tips leads towidening the gap between the blade tip and the surrounding shroud, andallowing the air to leak through, which can, in turn, degrade theefficiency of the turbine. Severe damage can even lead to cracking ofthe blade and possible blade failure.

Several approaches have been proposed to address the problem of theblade tip damage and air leakage within the airflow path. One approachis to apply a clearance sealing layer on the inner diameter of theshroud, so that the sealing layer can be abraded away by the blade tip,as disclosed in U.S. Pat. Nos. 4,540,336 and 4,280,975. Another approachis to incorporate a coating on the tip of the blade. The coating may actas a cutting edge (“squealer tip”) at the blade tip.

The coatings have been provided on the blade tips by a variety ofmethods, such as spraying, soldering, brazing, electroplating etc. Somecommonly used compressor coatings include alumina, cubic boron nitride,and chromium oxide coatings. One example is a coating containingtitanium carbide, applied onto a titanium alloy blade by means of asilver braze or a copper braze, as disclosed in EP-B1-0249092. Anotherexample involves sprayed carbide and boride coatings on a blade (U.S.Pat. No. 5,683,226).

However, currently used methods have several issues. For example,sprayed coatings can sometimes exhibit fatigue or other defects; andelectroplating may cause hydrogen embrittlement of the tips.Furthermore, conventionally available braze alloys have sometimes beenunable to meet the demand for low brazing temperature, high ductility,and low cost—characteristics that are necessary for severalapplications, such as aviation engines and industrial gas turbineengines.

Therefore, there is a need for a blade having a tip with improvedcharacteristics to meet performance requirements for turbineapplications. In some preferred embodiments, manufacturing the tipshould be less complicated and less expensive than existing tip coatingmethods.

BRIEF DESCRIPTION OF THE INVENTION

One embodiment of the invention is directed to a blade, e.g., a turbineengine blade. The blade may be a compressor blade generally used in theengines. The compressor blade includes a protective covering bonded to atip portion of the blade with a braze material. The braze materialincludes from about 1 weight percent to about 10 weight percent of anactive metal element, based on the total amount of the braze material.In one embodiment, a compressor rotor is provided that includes aplurality of the compressor blades.

Another embodiment of the invention is directed to a method for joininga protective covering to a tip portion of the compressor blade. Themethod includes the steps of disposing a braze material between asurface of a protective covering and a tip portion of the compressorblade, and joining the protective covering to the tip portion by heatingthe braze material to form a braze joint between the protective coveringand the tip portion. The braze material includes an active metal elementin an amount from about 1 weight percent to about 10 weight percentbased on the total amount of the braze material.

Yet another embodiment is directed to a method for repair of acompressor blade. The method includes the steps of removing a portion ofa worn or damaged tip of the blade, and providing a protective coveringon a tip portion of the blade, such that the protective coveringreplaces at least the removed portion of the tip of the blade. Theprotective covering is joined to the tip portion of the blade with abraze material, wherein the braze material comprises an active metalelement in an amount from about 1 weight percent to about 10 weightpercent based on the total amount of the braze material. The brazematerial is disposed between a surface of the protective covering and atip portion of a compressor blade, and heated to form a braze joint.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features, aspects, and advantages of the presentinvention will become better understood when the following detaileddescription is read with reference to the accompanying drawings in whichlike characters represent like parts throughout the drawings, wherein:

FIG. 1 is a schematic, cross-sectional view of a blade, in accordancewith some embodiments of the invention.

FIG. 2 is a schematic, cross-sectional exploded view of a portion of ablade, in accordance with some embodiments of the invention.

DETAILED DESCRIPTION

The present invention provides an improved blade tip for blades used ingas turbine engines, and particularly blades used in the compressorsection. As discussed in detail below, some of the embodiments of thepresent invention provide a blade incorporating a braze material (or abraze alloy) for joining a protective covering (which may also bereferred to as “protective cap”) to a tip portion of the blade, and amethod for the same. The invention further includes embodiments thatrelate to a method for repair of the blades. During service of theblade, a worn or damaged tip of the blade can be replaced by a newprotective covering. These embodiments advantageously provide animproved blade tip to address problems of tip damage and leakage fromthe gas flow path. Though the present discussion provides examples inthe context of blade tips for the compressor rotor, these processes canbe applied to any other component, for example, blades in other sectionsof the turbine.

Quite generally, in the interest of brevity of the discussions herein, aprotective coating or a protective covering or a protective capapplied/disposed on a tip portion of a blade may be referred to as a“tip” or “hard tip” of the blade.

Approximating language, as used herein throughout the specification andclaims, may be applied to modify any quantitative representation thatcould permissibly vary, without resulting in a change in the basicfunction to which it is related. Accordingly, a value modified by a termor terms, such as “about,” is not limited to the precise valuespecified. In some instances, the approximating language may correspondto the precision of an instrument for measuring the value.

In the following specification and claims, the singular forms “a”, “an”and “the” include plural referents unless the context clearly dictatesotherwise. As used herein, the terms “may” and “may be” indicate apossibility of an occurrence within a set of circumstances; a possessionof a specified property, characteristic or function; and/or qualifyanother verb by expressing one or more of an ability, capability, orpossibility associated with the qualified verb. Accordingly, usage of“may” and “may be” indicates that a modified term is apparentlyappropriate, capable, or suitable for an indicated capacity, function,or usage, while taking into account that in some circumstances, themodified term may sometimes not be appropriate, capable, or suitable.

The terms “comprising,” “including,” and “having” are intended to beinclusive, and mean that there may be additional elements other than thelisted elements. As used herein, the term “and/or” includes any and allcombinations of one or more of the associated listed items. Unlessotherwise indicated herein, the terms “disposed on”, or “disposedbetween” refer to both direct contact between components, objects,layers, and the like, or indirect contact, e.g., having interveninglayers therebetween.

In some embodiments, a blade includes a protective covering bonded to atip portion of the blade with a braze material. The blade may be acompressor blade, for example. The braze material includes from about 1weight percent to about 10 weight percent of an active metal element,based on the total amount of the braze material.

FIG. 1 schematically represents a blade 10. The blade 10 is exemplary ofthe type of compressor blades that are used in an industrial or aircraftturbine engine. The blade 10 includes a blade portion or an airfoil 12that acts against the incoming flow of air into the turbine engine, andaxially compresses the air flow. The blade 10 is usually mounted on adisk in a rotor (not shown) by a dovetail 14 which extends downwardlyfrom the airfoil 12, and engages a slot on the disk. The rotor may be anintegral bladed disk, or a disk with inserted blades. A platform 16extends longitudinally outwardly from the area where the airfoil 12 isjoined to the dovetail 14. The airfoil 12 includes a leading edge 18 anda trailing edge 20. The airfoil further includes a tip portion 21,having a tip edge 22 remote from the platform 16.

The airfoil 12 is relatively thin, as measured in a transverse direction(i.e. perpendicular to the platform). The dovetail 14 is relativelythick, as measured perpendicular to its direction of elongation.

A single blade 10 of the compressor section (not shown) of an engine isdepicted, though it should be understood that the blade 10 is one of anumber of blades in a compressor rotor. The number of blades may have asubstantially different shape and size, depending on many gas turbineengine models or applications. Typically, multiple blades are attachedto the rotor disk in adjacent circumferential positions. As mentionedpreviously, these blades are often circumscribed by a case (e.g., ashroud), which is positioned in close proximity to the radiallyoutermost tip portions (e.g., tip edges 22) of the blades (U.S.Publication 2012/0134786 and U.S. Pat. No. 5,059,095). The case servesto channel the air flowing through the compressor, so as to ensure thatthe bulk of the air entering the engine will be compressed within thecompressor section. A small gap is usually present between the blade tipand the case. Minimizing the gap promotes the efficiency of thecompressor section and the engine thereof.

A variety of materials may be used to form the blade 10. In oneembodiment, the compressor blade 10 includes an iron-based alloy.Suitable examples of such alloys include GTD 450, AISI 403, and Sermetelcoated AISI 403, which are described in a report entitled “Gas TurbineCompressor Operating Environment and Material Evaluation” by R. W.Haskell, which is incorporated herein, by reference.

In one embodiment, the compressor blade 10 includes titanium. Atitanium-base alloy having more titanium than any other element isusually desirable. One particular titanium-base alloy is known asTi-442, having a nominal composition, in weight percent, of about 4percent aluminum, about 4 percent molybdenum, about 2 percent tin, about0.5 percent silicon, and balance titanium. Another titanium-base alloyis known as Ti-811, having a nominal composition, in weight percent, ofabout 8 percent aluminum, about 1 percent molybdenum, about 1 percentvanadium, balance titanium. Another exemplary titanium-base alloy isknown as Ti 64, having a nominal composition, in weight percent, ofabout 6 percent aluminum, about 4 percent vanadium, balance titanium.Yet another exemplary titanium-base alloy is Ti62222, having a nominalcomposition, in weight percent, of about 6 percent aluminum; about 2percent tin, about 2 percent chromium, about 2 percent molybdenum; about2 percent zirconium; and balance titanium. Other suitable materials forthe blade 10 may include aluminum-based alloys, and nickel-basedsuperalloys such as M350 and IN718.

Referring to FIG. 1 again, the blade 10 includes a protective covering24 at the edge 22 of the tip portion 21. The protective covering 24forms the outer radial extremity of the blade 10. The protectivecovering 24 incorporates a hard material that can assist in preventingor at least minimizing abrasion between the tip 22 and the compressorcase (as discussed previously), and thus prevents/reduces degradation ofthe blade. Furthermore, the protective covering 24 may also serve topromote flow path sealing with the case, by creating a more tortuousflow path between the blade tip edge 22 and the case.

The protective covering 24, as used herein, is depicted in very generalform in the figures, for simplicity. The coating is meant to comprise apreform, a sheet, a wire, or a wafer that is capable of covering the tipedge 22 of the blade, in order to protect the edge from rubbing. Thecovering includes the hard material as discussed above, and may bejoined/bonded to the tip portion at the edge 22. In one embodiment, theprotective covering 24 includes a monolith of a suitable hard material.The material for the protective covering 24 may be prepared bycompacting the powder by mechanical means, casting, extrusion, molding,or additive manufacturing, followed by sintering (heat treatment). Thesintered protective covering can sometimes be machined into desiredshape and/or size.

A “hard material”, as used herein, usually refers to a material with ahardness value of at least about 15 GPa when measured by the Vickershardness test. These materials are highly resistive to various kinds ofpermanent shape change when a force is applied. The hardness of thesematerials generally depends on several factors, including ductility,elastic stiffness, plasticity, strain, strength, toughness, andviscosity. In some instances, the hardness of the material rangesbetween about 15 GPa and about 50 GPa, and in some specific instances,between about 20 GPa and about 40 GPa.

Suitable hard materials may include oxides, borides, carbides, nitrides,or a combination thereof. In some specific embodiments, the protectivecovering 24 includes alumina. In some embodiments, the protectivecovering 24 includes titanium boride, titanium carbide, or titaniumnitride. Other materials may include zirconium oxide, titaniumcarbonitride, titanium boronitride, chrome carbide, chrome nitride,chrome boride, molycarbide, or a combination thereof.

In some embodiments, abrasive particles may be added to the hardmaterial. One particular example of the abrasive material may be cubicboron nitride (CBN). The CBN may have an average particle diameterranging between about 25 microns and about 100 microns. Other abrasivematerials may include silicon carbide, cubic zirconia, and yttriumaluminum garnet.

The geometric profile of the protective covering 24 may be inconformance with the blade on which it needs to be adhered to. In otherwords, the protective covering 24 may be of a desired shape and size tocover the tip edge 22 of the blade, to prevent damage of the tipportion. In specific instances, the length and width of the protectivecovering 24 may be substantially equal to the length and width of thetip edge 22 of the blade 10. The covering 24 may be plane or curved inshape, with or without angles or rounding, in order to fit the shape ofthe tip edge 22. The thickness of the covering 24 (or height of the hardtip) may range from about 50 microns to about 1000 microns. In somespecific embodiments, the thickness of the covering 24 may range betweenabout 200 microns to about 500 microns.

The shape and size of several components discussed above with referenceto FIG. 1 are only illustrative for the understanding of the bladestructure; and are not meant to limit the scope of the invention. Theexact shape, size, and position of components (e.g., the protectivecovering 24) can vary to some degree.

As alluded previously, the protective covering 24 may be bonded to thetip portion 21 at the edge 22 by a braze joint or bond 26. The brazejoint 26 is formed by using a braze material that includes an activemetal element. An “active metal element”, as used herein, refers to areactive metal that has a high affinity to oxygen, and thereby reactswith the ceramic. A braze material or alloy containing an active metalelement can also be referred to as an “active braze alloy”, and thecorresponding technique may be referred to as “active brazing.” Activebrazing is a technique used to join a ceramic to a metal, or a ceramicto a ceramic. The active metal element undergoes a reaction with theceramic, when the braze alloy is in a molten state, and leads to theformation of a thin reaction layer on the interface of the ceramic andthe braze alloy. The thin reaction layer allows the braze alloy to wetthe ceramic surface, resulting in the formation of a ceramic-ceramic ora ceramic-metal joint/bond, which may also be referred to as “activebraze joint or seal.”

In some embodiments, the braze material includes a silver-based alloy, acopper-based alloy, or a gold-based alloy. In general, the amount ofsilver, copper, or gold balances the alloy based on the amounts of theother constituents. As discussed previously, the braze alloy includes anactive metal element. A variety of suitable active metal elements may beused to form the active braze alloy. The selection of a suitable activemetal element mainly depends on the chemical reaction with the ceramic(e.g., alumina) to form a uniform and continuous reaction layer, and thecapability of the active metal element of forming an alloy with a baseelement (e.g. nickel). The active metal element for embodiments hereinis often titanium. Other suitable examples of the active metal elementinclude, but are not limited to, zirconium, hafnium, and vanadium. Acombination of two or more active metal elements may also be used.

The presence and the amount of the active metal may influence thethickness and the quality of the thin reaction layer, which contributesto the wettability or flowability of the braze alloy, and therefore, thebond strength of the resulting joint. A high amount (for example,greater than about 20 weight percent) of the active metal element liketitanium in a braze alloy has been thought to be technicallyundesirable, due to the formation of brittle compounds with severalalloying elements present in the blade alloy. The formation of thesebrittle compounds may initiate cracks at the braze joint. However, itwas observed that a small amount (for example, less than about 10 weightpercent) of the active metal element is generally suitable for improvingthe wetting of the ceramic surface (e.g., the protective covering 24),and forming the thin reaction layer (e.g., less than about 10 microns)to form a bond with the ceramic covering (e.g., alumina). In someembodiments, the active metal element is present in an amount rangingfrom about 0.1 weight percent to about 10 weight percent, based on thetotal weight of the braze alloy. A suitable range is often from about 1weight percent to about 6 weight percent.

The braze alloy composition may further include at least one additionalelement. The additional element may provide adjustments in severalrequired properties of the braze alloy, for example, the coefficient ofthermal expansion, liquidus temperature, brazing temperature, corrosionresistance, and the strength of the braze alloy. In one embodiment, theadditional element can include, but is not limited to, chromium, cobalt,nickel, aluminum, tin, zinc, molybdenum, germanium, silicon, or acombination thereof. With respect to the amount, the braze alloyincludes up to about 10 weight percent (e.g., about 0.1%-20%) of theadditional elements, based on the total weight of the braze alloy. Insome embodiments, the braze alloy includes from about 0.1 weight percentto about 2 weight percent of molybdenum, based on the total weight ofthe braze alloy. Some examples of the braze alloy may include CopperABA® (92.75Cu-2Al-3Si-2.25Ti), 63Ag-35.25Cu-1.75Ti, and82Au-15.5Ni-1.75V-0.75Mo.

Some embodiments provide a method for joining a protective covering to ablade tip. The method includes the steps of introducing the braze alloybetween the protective covering and the blade tip to form a brazingstructure. (The braze alloy could be deposited on one or both of themating surfaces, for example, as also described below). The brazingstructure can then be heated to form an active braze joint or sealbetween the protective covering and the blade tip. In one embodiment,the protective covering includes a ceramic; and the blade includes ametal. The braze alloy composition includes copper, gold, or silver, andan active metal element. At least one additional alloying element, suchas chromium, cobalt, nickel, aluminum, tin, zinc, molybdenum, germanium,silicon, or a combination thereof, may further be added. Theconstituents of the braze alloy and their respective amounts (andproportions) are described above.

For joining the protective covering 24 to the blade tip 22, the brazealloy may be introduced between them. For example, referring to FIG. 1,the braze alloy may be disposed between the blade tip edge 22 and thecovering 24. In some instances, a facing layer of the braze alloy may beapplied on the tip edge 22, a surface of the covering 24, or both of thesurfaces being joined. The thickness of the alloy layer may be in arange between about 5 microns and about 100 microns, and in somespecific embodiments, between about 15 microns and about 50 microns. Thelayer may be deposited or applied on one or both of the surfaces to bejoined, by any suitable technique, e.g. by a printing process or otherdispensing processes. In some instances, a foil, a sheet, wire, or apreform may be suitably positioned for bonding the surfaces to bejoined.

The method further includes the step of heating the brazing structure.The entire assembly (or a brazing structure) is usually heated at asuitable brazing temperature (for example, about 1100 degrees Celsius);and the braze alloy melts and flows over the surfaces. The brazingtemperature is generally less than the melting temperatures of thecomponents to be joined, and higher than the liquidus temperature of thebraze alloy. The heating can be undertaken in a controlled atmosphere,such as ultra-high pure argon, hydrogen and argon, ultra-high purehelium; or in a vacuum. To achieve good flow and wetting of thesurfaces, the assembly is often held at the brazing temperature for afew minutes after melting of the braze alloy. This period may bereferred to as the “brazing time”. During the process, a load can alsobe applied.

During brazing, the alloy melts and the active metal element (orelements) present in the melt reacts with the ceramic and forms a thinreaction layer 28 at the interface of the protective covering 24 and thebraze alloy (described previously), as illustrated in FIG. 2. Thethickness of the reaction layer may range from about 0.1 micron to about2 microns, depending on the amount of the active metal element availableto react with the ceramic, and depending on the surface properties ofthe ceramic protective covering. In a typical sequence, the brazingstructure is then subsequently cooled to room temperature; with theresulting active braze hermetic joint 26 situated between the twocomponents. In some instances, rapid cooling of the brazing structure ispermitted.

When articles such as several components of gas turbines are serviced,the protective coatings, and tip coverings or tip caps (or hard tips)are usually removed to permit inspection and possible repair of the tipcaps and the underlying substrate, followed by re-coating the blade andthe tip of the blade. Removal of the coatings and tip caps is typicallycarried out by immersing the component in a stripping solution. Avariety of stripping techniques are currently available for removingdifferent types of coatings from metal substrates. The techniquesusually must exhibit a considerable amount of selectivity. In otherwords, they must remove only intended materials, while generallypreserving the article's desired structures.

Some embodiments of this invention relate to a method for repair of acompressor blade. During repair, a worn or damaged tip of the blade canbe removed, and replaced by a protective covering or cap applied over atip portion of a blade (e.g, a compressor blade). The term, “replace” asused herein means that the protective covering occupies the region onthe tip edge of the blade formally occupied by the removed portion, orat least a desired portion of the region, if full coverage is not neededin the repair. The first step of the method includes the removal of aportion of the worn or damaged tip of the blade. Any suitable method canbe employed to remove the damaged tip. In some embodiments, anelectrochemical method can be used for the removal of the damaged tip.The electrochemical treatment is usually followed by de-smutting andrinsing steps. The replacement (or new) protective covering can then beapplied to the tip portion of the blade by the active brazing method, aspreviously discussed in detail. In one embodiment, a sintered protectivecovering may be joined to the tip portion of the blade.

Embodiments of the present invention provide advantages to leverage arelatively inexpensive, simple, and rapid process to hermetically joinand manufacture the blade tip, as compared to currently availablemethods. The resulting blade tips are very strong, reliable, andchemically stable in the corrosive environment. For example, the tensilestrength of the active braze joint often ranges between about 100 Mpaand about 200 MPa for a ceramic to metal butt joint. Moreover, anadditional advantage provided by active brazing is the ability toachieve the joint in single step, and thus to manufacture the blade tipin fewer steps, and simpler steps, as compared to known multi-step,cumbersome manufacturing processes. In brief, active brazing simplifiesthe joining process, and improves the reliability and performance of theblade tip.

In some embodiments, a compressor rotor comprises a plurality of blades.The plurality of blades is arranged circumferentially on the rotor. Insome embodiments, every blade of the rotor includes the protectivecovering 24 (e.g., hard tip) (as discussed above). In some embodiments,a few (not all) blades include the hard tip, which are usually referredto as “cutter blades.” The purpose of the cutter blade is to engageshroud material located on the inner surface of the casing, to furtherminimize any leakage around the blade tip. The cutter blade includingthe hard tip cuts the shroud material as the blade rotates, providing anincursion into the shroud material. The hard tip serves to wear-form aseal track in the shroud, resulting in a virtual seal between the bladesand the shroud which prevents gases from bypassing the rotor assembly.The advantage of the cutter blade is that only a small number of thecompressor blades need to be given the hard protective covering, whilethe rest can be left bare. Thus, for some embodiments, the cost ofdisposing protective coverings on every blade, and related processing(tip grinding to the precise height and profile) can be avoided.Further, the cutter blades can be made deliberately taller than the restof the compressor blades, so that only the covered or capped cutterblades make substantial contact with the shroud material. By cuttinginto the shroud and creating a trench, the cutter blades protect therest of the blades from rubbing, and reduce overall damage.

While only certain features of the invention have been illustrated anddescribed herein, many modifications and changes will occur to thoseskilled in the art. It is, therefore, to be understood that the appendedclaims are intended to cover all such modifications and changes as fallwithin the true spirit of the invention. Furthermore, all of thepatents, patent applications, articles, and texts which are mentionedabove are incorporated herein by reference.

1. A compressor blade, comprising a protective covering bonded to a tipportion by a braze material, wherein the braze material comprises fromabout 1 weight percent to about 10 weight percent of an active element,based on the total amount of the braze material.
 2. The compressor bladeof claim 1, formed of a material comprising titanium, iron, or an alloycontaining one or more thereof.
 3. The compressor blade of claim 1,formed of a titanium-based alloy.
 4. The compressor blade of claim 1,formed of stainless steel.
 5. The compressor blade of claim 1, whereinthe protective covering comprises a hard ceramic material.
 6. Thecompressor blade of claim 5, wherein the protective covering comprisesaluminum oxide, zirconium oxide, titanium boride, titanium carbide,titanium nitride, titanium carbo-nitride, titanium boro nitride, chromecarbide, chrome nitride, chrome boride, molycarbide, or a combinationthereof.
 7. The compressor blade of claim 1, wherein the protectivecovering is present in the form of a preform, a sheet, a wafer, or awire.
 8. The compressor blade of claim 1, wherein the protectivecovering has a thickness in a range from about 50 microns to about 1000microns.
 9. The compressor blade of claim 1, wherein the braze materialcomprises copper, silver, gold, zinc, manganese, tin, nickel,molybdenum, or an alloy containing one or more thereof.
 10. Thecompressor blade of claim 1, wherein the amount of the active metalelement is in a range from about 2 weight percent to about 6 weightpercent, based on the total weight of the braze material.
 11. Thecompressor blade of claim 1, wherein the active metal element comprisestitanium, hafnium, vanadium, or zirconium.
 12. A compressor rotor,comprising a plurality of blades in accordance with claim
 1. 13. Amethod, comprising: disposing a braze material between a surface of aprotective covering and a tip portion of a compressor blade, wherein thebraze material comprises an active metal element in an amount from about1 weight percent to about 10 weight percent, based on the total amountof the braze material; and joining the protective covering to the tipportion by heating the braze material to form a braze joint between thetip portion and the protective covering.
 14. The method of claim 13,wherein the protective covering comprises a hard ceramic material. 15.The method of claim 14, wherein the protective covering comprisesaluminum oxide, zirconium oxide, titanium boride, titanium carbide,titanium nitride, titanium carbo-nitride, titanium boro nitride, chromecarbide, chrome nitride, chrome boride, molycarbide, or a combinationthereof.
 16. The method of claim 13, wherein the protective covering ispresent in the form of a preform, a sheet, a wafer, or a wire.
 17. Amethod for repair of a compressor blade, comprising the steps of: (i)removing a portion of a worn or damaged tip of the blade from a tipportion of the blade; and (ii) providing a protective covering on thetip portion of the blade such that the protective covering replaces atleast a portion of the removed portion of the tip of the blade, andwherein providing the protective covering comprises the steps of:disposing a braze material between a surface of the protective coveringand a tip portion of a compressor blade, wherein the braze materialcomprises an active metal element in an amount from about 1 weightpercent to about 10 weight percent based on the total amount of thebraze material; and joining the protective covering to the tip portionby heating the braze material, to form a braze joint between the tipportion and the protective covering.